Rather than being better than mankind by virtue of their spiritual nature, we find on examination that the demons are actually much worse off, since they exist in a state of permanent wickedness.
They would be intermediate if they held one of their qualities in common with the one party, and the other with the other, as man is a kind of mean between angels and beasts,—the beast being an irrational and mortal animal, the angel a rational and immortal one, while man, inferior to the angel and superior to the beast, and having in common with the one mortality, and with the other reason, is a rational and mortal animal. So, when we seek for an intermediate between the blessed immortals and miserable mortals, we should find a being which is either mortal and blessed, or immortal and miserable.Again, we can see Augustine leading us towards the Incarnate Christ as the only true Mediator between God and man.
But, if men are mortal can they ever truly enjoy eternal blessedness?
Yes, because the Immortal God has put on mortality and so raised us up from our limitedness and sinfulness and reconciled us to God
...that one beatific good, to obtain which we need not many but one Mediator, the uncreated Word of God, by whom all things were made, and in partaking of whom we are blessed. I do not say that He is Mediator because He is the Word, for as the Word He is supremely blessed and supremely immortal, and therefore far from miserable mortals; but He is Mediator as He is man, for by His humanity He shows us that, in order to obtain that blessed and beatific good, we need not seek other mediators to lead us through the successive steps of this attainment, but that the blessed and beatific God, having Himself become a partaker of our humanity, has afforded us ready access to the participation of His divinity. For in delivering us from our mortality and misery, He does not lead us to the immortal and blessed angels, so that we should become immortal and blessed by participating in their nature, but He leads us straight to that Trinity, by participating in which the angels themselves are blessed. Therefore, when He chose to be in the form of a servant, and lower than the angels, that He might be our Mediator, He remained higher than the angels, in the form of God,—Himself at once the way of life on earth and life itself in heaven.Christ alone can mediate between God and man, not even the good angels can perform this office. Over and over Augustine drives home the point that salvation comes through Christ alone; it is to Him and only to Him that we must turn in order to partake of the life everlasting.